1. Field of the Invention
The present invention relates to electrical circuits for starting and restarting high intensity discharge (HID) lamps and, more particularly, to a novel circuit for producing high ignition voltage for starting an HID lamp.
2. Related Art
It is well known in the art that HID lamps commonly operate in a starting mode, an operating or steady state mode, and a restrike mode. During the starting mode, a relatively high value of starting voltage (i.e., ignition voltage) of about 25 KV is applied across electrodes of a lamp to first place the gases of the lamp into a suitable ionized condition for striking or initiating a glow breakdown state.
Automotive HID ballasts typically consist of a DC to DC converter, an H-bridge and an ignitor circuit, for example, see U.S. Pat. No. 4,904,907 to Allison.
Prior art ignitor circuits generally consist of a pulse transformer capable of producing a high voltage pulse and a drive circuit consisting of a capacitor, a spark gap and an resistor. For example, U.S. Pat. No. 5,036,256 to Garrison discloses an arc discharge ballast suitable for automotive applications consisting of a DC to DC converter, step-up transformer, rectifier and voltage doubler, energy storage circuit, and ignition transformer. In the Garrison circuit, a capacitor, resistor and spark gap form a relaxation oscillator capable of periodically pulsing the ignition transformer. The maximum charge voltage on the capacitor is limited to the supply voltage applied to the input of the relaxation oscillator.
In general, higher charge voltages are desirable since the energy stored in the capacitor is 1/2 CV.sup.2. Indeed, higher charge voltages permit a reduction in the capacitor size while maintaining a constant amount of stored energy. One method of increasing the charge voltage above a supply voltage is to use a voltage doubler operated from an H-bridge as described in U.S. Pat. No. 5,534,753 to Blom. In this way, charge voltages of up to twice the supply voltage may be obtained. In a voltage doubler, the effective capacitance is halved since there are two energy storage capacitors in series.
Unfortunately, the prior art arc discharge ballast circuits are disadvantageous for a number of reasons which include a limitation in the reduction of the energy storage capacitor, excessive voltage pulsing of the ignition transformer and asynchronous operation of the ignition pulse circuit with respect to the H-bridge circuit.
Accordingly, there is a need in the art for an improved ignition circuit for automotive HID lamps which permits doubling the pulse voltage, reducing the size of the energy storage capacitors by more than one half, synchronization of the ignition voltage pulse with the H-bridge circuit, and limiting the number of ignition pulses per half cycle of the H-bridge.